I don't think the TI is a boat that professional thieves would target, it's too niche and they have better options. But if you're concerned, you can lock the trailer to your car with a high-quality trailer lock and then chain the TI to the trailer with a security chain and high-security lock such as
this.
No lock and chain will stop a determined professional, but this will slow them down enough for them to probably look for an easier target. They want to be quiet, stealthy, and fast. High-security locks and chains take time and make noise to cut, so unless the target is really worth it, they'll move on to easier prey.
As far as other items such as the sail, electronics, etc., you should take them inside if staying overnight in a hotel or lock whatever items you can inside the car and out of sight. That's about the best you can do. Insurance is also available to cover any serious losses. My TI is covered for theft on my home owner's insurance, but in five years no one has ever tried to steal it or its contents.